Bridge conjugation

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

The interaction of a complex anion SbY5Z with excessive Lewis acid SbX5 leads to the formation of a ligand-bridged (conjugated 1)) anion (see part 4.2.2) according to... 

Effect of Bridge Conjugation on Electron Transfer Dynamics... 

In view of recent reports, it seems prospective to use the reaction of trans sily-lation for the synthesis of other types of unsaturated compounds, particularly for the synthesis of novel organosilicon starburst compounds having a silicon-bridged -conjugated structure which is expected to have potential optoelectronic properties. The effective functionalization of l,3,5-tris(dimethylvinylsi-lyl)benzene by the respective reactions with 1,4-divinylbenzene leads to the formation of a dendrimer (Fig. la) [55]. 

In systems in which the donor and acceptor centers are in direct contact with each other or connected by a conducting bridge (conjugated bonds), electron transfer rates are very fast (kET = 10"13 -10 12 s 1). The transition occurs markedly slower when the donor-acceptor mutual orientation is not favorable for positive orbital overlap and, therefore, the electron coupling V is small. 

The synthesis of all these molecules has been based on a synthon approach, the vital starting compound, 1,6-diformylcycloheptatriene, being- prepared by straightforward steps from the readily available cycloheptatriene [195]. This may in turn be converted into other bridged conjugated dialdehydes which serve as precursors for the methanoannulenes, e.g. [182,193,195] ... 

The rational synthesis of anionic, neutral, and cationic dinuclear palladium complexes containing bridging conjugated dienes is shown in Scheme 32 for butadiene, and consists of the formal redox condensation of Pd(0) and Pd(ii) complexes in the presence of the conjugated diene to give Pd(l) dimers. In addition, some complexes with isoprene and with 1,3-cyclohexadiene were made. The isoprene complexes consisted of the expected pair of regioisomers in the case of the less symmetric neutral molecules. The crystal structures of one neutral complex... 

Another synthesis of a bridged hydrocarbon takes advantage of high elearon release from the /wra-position of phenolate anions, which may be used to transform the phenol moiety into a substituted cross-conjugated cyciohexadienone system (S. Masamune, 1961, 1964). 

Ghromophore Interaction. A great number of systems containing several polymethine chromophores exist. As the simplest objects of this sort, dyes with coupled chromophores, or bis-dyes, can be considered (3,10) as follows (15), where M is either a conjugated or a nonconjugated bridge. 

Nonconjugated perfluorocyclohepta-l,4-diene is oxidized to the corresponding diepoxide by sodium hypobromite [17] (equation 36), whereas the conjugated- 1,3-diene gives a mixture of 1,2-monoepoxide and bridged 2,3 1,4-diepox-ide [IT] (equation 36). 

Conjugated dienamines were found to give predominantly double four-membered-ring adducts as well as a small amount of the six-membered-ring adduct (466,468). This important result indicates preferred attack at the terminal carbon of the dienamine system (in contrast to alkylation, for instance) in the generation of an initial zwitterionic intermediate. Addition of sulfonyl chloride and triethylamine to a homocyclic dienamine gave only the bridged product (446). 

The reaction of relatively simple starting materials, coumarin 40, piperidone 37 and ammonium acetate, leads in a single step to the complex bridged tetracyclic compound 44. The reaction can be rationalized by assuming formation of the iminc 38 from reaction of 37, with ammonia. Conjugate addition of the eneamine-like tautomer 39 to the excellent Michael acceptor 40 will... 

The versatility of poly(phenylcne) chemistry can also be seen in that it constitutes a platform for the design of other conjugated polymers with aromatic building blocks. Thus, one can proceed from 1,4- to 1,3-, and 1,2-phenylene compounds, and the benzene block can also be replaced by other aromatic cores such as naphthalene or anthracene, helerocyclcs such as thiophene or pyridine as well as by their substituted or bridged derivatives. Conceptually, poly(pheny ene)s can also be regarded as the parent structure of a series of related polymers which arc obtained not by linking the phenylene units directly, but by incorporation of other conjugated, e.g. olefinic or acetylenic, moieties. 

The synthesis of homoporphyrins in which one methine bridge of the parent porphyrin is extended to a two-carbon bridge is one of the earliest and simplest examples of an expanded porphyrin.3a b>4 The synthesis of homoporphyrin 4 is based on the ability of N-alkylated porphyrin 1 to undergo nickel-induced rearrangement to form an expanded macrocycle 2. De-metalation of 2 by means of concentrated hydrochloric acid yields the cyclically conjugated [20]porphyrin(2.1.1.1) 4 and a macrocyclic side product 3 in which the cyclic conjugation is interrupted. 

---